The quadratic hashing issue is a non-problem.
Any miner that creates a block that takes inordinate time to validate will find himself bankrupted by other miners who continue hashing on the same parent to find a peer solved block. Such a peer solved block will validate well before the aberrant block, leading to the aberrant block being orphaned. 'Problem' solved. With the incentives as they exist today. Unchanged.
Any miner that creates a block that takes inordinate time to validate will find himself bankrupted by other miners who continue hashing on the same parent to find a peer solved block. Such a peer solved block will validate well before the aberrant block, leading to the aberrant block being orphaned. 'Problem' solved. With the incentives as they exist today. Unchanged.
I love the semantic hair split between "issue" and "problem" that you start off with. Great spin. Very Comical Ali of you.
Spin? WTF? No spin here.
I could have used the word 'aspect' instead of 'issue'. Or 'attribute'. Or some other. Wouldn't have made much difference.
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But how can you predict exact future scenarios like that? Are you psychic? Do you have visions or hear angels?
It is not a 'prediction', it is pointing out an essential element of the incentives already baked in to Bitcoin. I know you know that Bitcoin only works _at_all_ because the incentives are such that 'doing the right thing' is profitable. Right? Well, this is just one more instance of doing the right thing being profitable.
I'll admit that I don't know if any significant miners currently implement this policy. Then again, aberrant blocks -- in the sense that they take a huge time to validate -- are rare. Indeed, I am aware of only one ever being created. But if such blocks ever became A Thing, rational miners will implement the mode I describe above. Because orphaning such aberrant blocks by others guarantees increased profitability.
Sure, we might choke down several aberrant blocks in the meantime. But that won't bring the network down. It merely forestalls block completion time in the same manner that a variance outlier does. We've choked down at least one of these blocks before -- indeed one crafted intentionally to max out validation time -- to no ill effect.
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The rest of us simply don't know the specifics about the situation (which seems to be both empirical and theoretical!) you are describing.
Don't blame me for your being unable to game out this perfectly rational scenario. It is the only such scenario that makes sense in the case of such aberrant blocks.
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Doesn't the percent of total mining power the putative attack block creator determine the likelihood of his attack fork will be reclaimed by the defending chain?
Of course. A 51% attacker always gets to define the chain. What's your point?
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Doesn't variance also play a non-computable role in determining whether the attack block-based or defending chain-based side will win?
Of course. These things are all probabilistic. But the incentives are aligned to render this (::ahem::!) attribute a non-problem.
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If the quadratic hashing issue is a truly a "non-problem" then why did Gavin write an unforgivably kludgy, quick and dirty, non-futureproof workaround for it? 
Because Gavin is a pragmatist that endeavored to give the users what they wanted, as long as it did not cause any actual harm? I dunno - you'd need to ask him.
Incidentally, I'm not aware of any 'unforgivably klugy' such work by him. To which pull req are you referring?